In a conventional manner to make a lithographic printing plate, a photosensitive lithographic printing plate having a thin coating of photosensitive composition on an aluminum plate is exposed imagewise and then developed. The aluminum plate to be used as a support for the lithographic plate generally undergoes a surface roughening treatment using a mechanical method, such as a brush graining method or a ball graining method, an electrochemical method such as an electrolytic graining method, or a combination of mechanical and electrochemical methods, and thereby it comes to have a rough surface like a pear skin. The thus surface-roughened aluminum plate is etched with an aqueous solution of acid or alkali and then anodized. Further, the anodized aluminum plate may receive some treatment for improving water wettability, if needed. On the support prepared in the aforementioned manner, a photosensitive layer is provided to form a photosensitive lithographic printing plate (the so-called presensitized plate). In general such a presensitized plate is subjected successively to imagewise exposure, development and gumming processes to be made into a lithographic printing plate. This lithographic printing plate is mounted in a press, and printing operations are carried out.
As the photosensitive lithographic printing plates to provide lithographic printing plates, there are those of a positive working type and a negative working type.
The positive working photosensitive compositions which have widely been used are compositions comprising o-quinonediazide compounds. Such a photosensitive compound alone or a mixture with an alkali-soluble resin, such as a novolak-type phenol or cresol resin, is coated on a support. In a case where the support used has a water receptive surface, the exposed area of the coating is easily removed with an alkali developer since it changes to soluble in alkali due to decomposition of the o-quinonediazide compound, and thereby the water receptive surface of the support is bared. This bared area of the support receives water but repels ink; while the unexposed area remaining as an image is lipophilic, and so it can receive ink.
With respect to the negative working photosensitive compositions, there are many cases in which a diazonium salt, an azide compound or a photopolymerizing compound is used as photosensitive component. Such a photosensitive component is coated on a support singly or as a mixture with an additive such as an appropriate resin. When a support having a water receptive surface is used, the unexposed area of the coating is removed with a developer to reveal the water receptive surface of the support. This area receives water and repels ink. On the other hand, the area cured by exposure remains as an image upon development, and can receive ink because of its lipophilic properties.
As a method of preparing a support having a water receptive surface, the following are known: the method disclosed in U.S. Pat. No. 3,181,461 wherein the anodized layer is treated with an aqueous alkali metal silicate, the method disclosed in JP-A-2-185493 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") wherein the anodized aluminum plate is treated with an aqueous solution containing an alkali metal silicate and a hydroxide, the method described in JP-B-46-35685 (the term "JP-B" as used herein means an "examined Japanese patent publication") wherein the anodized aluminum plate is treated with an aqueous solution of polyvinylphosphonic acid, the method described in JP-A-60-194096 wherein the anodized aluminum material is treated with an aqueous solution of alkali metal silicate, and then with an aqueous solution containing at least one organic polymer having vinylphosphonic acid units and/or vinylmethylphosphinic acid units, the method disclosed in WO 9509384 wherein water wettability is given to an aluminum plate by treating the plate with an aqueous solution of polyvinylphosphonic acid neutralized to pH 2.5-6.6 with an alkali, and the method disclosed in WO 9509086 wherein the mechanically or/and electrically grained aluminum plate is treated with an aqueous solution of polyvinylphosphonic acid neutralized to pH 2.5-6.6 with an alkali to pH 2.5-6.6 to acquire water wettability.
Those methods for conferring water wettability on a support surface have some problems to solve. For instance, poor inking (water log) due to excess of water and ink spreading in the shadow part of an image can occur due to an excess of ink on the lithographic printing plate at the same time depending upon positions along the width direction in a printing machine, thereby failing in providing prints of good quality. In still another case where a great number of copies are printed, stains are generated on the prints with the progress of printing operations since the non-image area of the plate is gradually rendered hydrophobic by the printing ink. In order to recover from this hydrophobic condition, a cleaner containing as main components a support etching agent, such as phosphoric acid, and an ink dissolving agent, such as an aliphatic hydrocarbon, is usually employed. However, the use of such a cleaner causes deterioration of impression capacity in negative working presensitized plates formed using as the support a material which has undergone anodic oxidation and then treatment with an aqueous solution of alkali metal silicate, because the adhesion between the photosensitive layer and the support is weakened by the cleaner. In other negative working presensitized plates formed using as the support an aluminum plate which has undergone anodic oxidation and then treatment with an aqueous solution of polyvinylphosphonic acid adjusted to pH 2.5 or higher, the cleaner used has no bad effect on their impression capacity, but in the case where water is used in a reduced amount as compared with a standard amount, the tendency of causing stains by ink spreading in the image area and stains in the non-image area on prints is increased, and when the amount of water is increased after the stains generate in image area or non-image area, the tendency of causing the prints to suffer from the stains (so-called "water-ink balance scum") is increased. In still another case as described in Example 1 of JP-A-60-194096, where the negative working presensitized plate is formed using the support which has been anodized and then treated with an aqueous solution of alkali metal silicate and further undergone after-treatment with an aqueous solution of polyvinylphosphonic acid, the use of the cleaner causes deterioration of impression capacity, in analogy with the case of using the support treated with an aqueous solution of alkali metal silicate alone.